Preventive maintenance method and apparatus of a structural member in a reactor pressure vessel

ABSTRACT

A preventive maintenance method and apparatus for a structural member in a reactor pressure vessel according to the present invention reduce a tensile residual stress on a surface of the structural member by impinging a water jet from a nozzle onto a plane surface of a deflector to thereby change direction of flow of the water jet, and impinging the water jet after being deflected onto the surface of the structural member. This method and apparatus are applicable to a narrow space portion, and can improve a residual stress on the surface of the structural member and can also prevent damage such as stress corrosion cracking.

BACKGROUND OF THE INVENTION

The present invention relates to a preventive maintenance method andapparatus for reducing a tensile residual stress on a surface of astructural member in a reactor pressure vessel (RPV) by discharging aliquid (water) jet to the surface of the structural member, therebypreventing occurrence of stress corrosion cracking (SCC). Particularly,the present invention relates to a method and apparatus suitable forreducing the tensile residual stress of a weld portion and a weldheat-affected zone located in a narrow space or an inaccessible portion.

A water jet peening (WJP) method is known as a method that adds acompressive residual stress to a surface layer of a metal material. Inthe WJP method, a nozzle is set opposite to the metal material in water,and a water jet containing cavitation bubbles is discharged from thenozzle toward the metal material in the water. When the water jetcollides with (or impinges on) a surface of the metal material, thecavitation bubbles are collapsed by an axial dynamic pressure. When thecavitation bubbles are collapsed, an impact pressure is produced by awater-hammering effect, and this impact pressure strikes the surface ofthe metal material to add the compressive residual stress.

The first prior art as to the WJP method is disclosed in Japanese PatentLaid-open No. Hei 4-362124. In this method, a WJP is performed bydischarging a water jet containing cavitation bubbles from a nozzlewhich is directed to a metal material in water, and by impinging thewater jet on a surface of the metal material while moving the nozzlealong the metal material.

The second prior art applicable to an inner surface of a tube with asmall diameter is disclosed in Japanese Patent Laid-open No. Hei10-76467. In this case, a high speed liquid jet containing cavitationbubbles is discharged from a nozzle which is directed to an axialdirection of a tube, and a conical baffle body which gradually reduces across-sectional area of a flow passage in the tube and a columnar bafflebody located adjacent to the conical baffle body are coaxially providedon a downstream side from the nozzle.

In a region where the conical baffle body is provided, the cavitationbubbles are collapsed limitedly near an inner surface of the tubebecause a peripheral pressure of the jet gradually increase by arestriction effect of the flow passage. Fine cavitation bubbles, whichare not collapsed in the above region, become nuclei of other cavitationbubbles in a local low pressure area produced by a separation phenomenonof the jet flow at a transition corner from the conical baffle body tothe columnar baffle body, and generate secondary cavitation bubbles.This secondary cavitation bubbles are collapsed at a downstream sidefrom the corner. This document also shows one example of the conicalbaffle body which has an apex angle of 60° in a longitudinal crosssection.

The above two methods are intended to subject a metal surface to peeningtreatment using collapse pressures of cavitation bubbles so as toconvert a tensile residual stress which initially presents in a surfacelayer of the metal material into a compressive residual stress.

However, the first WJP method is carried out by discharging the jet fromthe nozzle provided opposite to the metal material while moving thenozzle along the metal material. Accordingly, this WJP method isdifficult to apply to a narrow space portion such as an outer surface ofa core shroud and an inner surface of a tubular structure with a smalldiameter such as an in-core monitor (ICM) housing in a RPV.

The second WJP method is applicable to the ICM housing, but can notapply to the narrow space portion such as the outer surface of the coreshroud. Further, in a case that the second WJP method applies to the ICMhousing, the restriction effect of the flow passage becomes almostuniform in a peripheral direction in the region where the conical bafflebody is provided, but an effect of generating a peeling flow bycollision of the jet is not obtained.

Therefore, since the cavitation bubbles do not grow so largely, a localimpact pressure (collapse pressure) applied to the inner surface of thetube is restricted (limited). The peeling flow is generated at thetransition corner from the conical baffle body to the columnar bafflebody, but since the corner has an obtuse angle in a longitudinal crosssection, strength of the peeling flow is also restricted.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a preventivemaintenance method and apparatus of a structural member in a reactorpressure vessel (RPV), capable of applying to a narrow space portionsuch as an outer surface of a core shroud and an inner surface of atubular structure with a small diameter such as an in-core monitor (ICM)housing in the RPV filled with core water, and also capable of producingcavitation bubbles with high collapse pressures, and improving aresidual stress on a surface of the structural member by collapsing thecavitation bubbles at a desired surface of the structural member,thereby preventing a damage such as stress corrosion cracks (SCC).

A water jet, which has collided with (or impinged on) a wall surface ina direction substantially perpendicular thereto and has influenced bythe collision (or the impingement), is particularly called a collisionjet. In such a collision jet, a separation flow more violent (stronger)than that caused by collision with a portion having a simplediscontinuous shape is produced by an vortex flow and a turbulent flow.

As a result, when the water jet collides with the wall surface, part ofcavitation bubbles contained in the water jet collapse at the wallsurface. But remaining fine cavitation bubbles grow near the wallsurface and new cavitation bubbles are generated near the wall surface.The present invention is based on such a feature of the collision jet.

In accordance with the present invention, a preventive maintenancemethod of a structural member in a reactor pressure vessel for reducinga tensile residual stress on a surface thereof, has the steps ofimpinging a water jet from a nozzle onto a plane surface of a deflectorto thereby change direction of flow of said water jet, and impinging thewater jet after being deflected onto the surface of the structuralmember.

In this case, since it need not to direct the nozzle to the surface ofthe structural member, if a spatial width of a narrow space portion (oran inner diameter of a tube with a small diameter) is larger than anouter diameter of the nozzle, this method is applicable to the narrowspace portion (or the tube with the small diameter).

Further, since the water jet from the nozzle becomes a collision jetincluding a strong separation flow and a strong vortex flow by theimpingement on (or collision with) the plane surface of the deflector,cavitation bubbles contained in the collision jet grow largely (becomelarge). As a result of a combination of this effect and a strongwater-hammering effect on the surface of the structural member, thecavitation bubbles in the collision jet give high collapse pressures tothe surface of the structural member when the cavitation bubblescollapse on the surface of the structural member. That is, a highcompressive residual stress can be added to the surface of thestructural member. Accordingly, it can be possible to improve a residualstress on the surface of the structural member and also prevent a damagesuch as SCC.

Preferably, a distance between the nozzle and the plane surface of thedeflector is at most 100 times (preferably at most 50 times) as large asa hole diameter of the nozzle. In this case, since the water jetcollides with the plane surface of the deflector before fine cavitationbubbles contained in the water jet become large and velocity of thewater jet becomes low, it is possible to reduce the amount of thecavitation bubbles collapsed by the collision with the plane surface ofthe deflector, and also make the fine cavitation bubbles largely grow bya collision effect with the plane surface of the deflector. Accordingly,the cavitation bubbles having high collapse pressures can be collapsedon the surface of the structural member.

Preferably also, an angle formed between a central axis passing throughan opening of the nozzle and the plane surface of the deflector is in arange of 10° to 90°, preferably in a range of 40° to 90°, morepreferably in a range of 60° to 90°. In this case, since the collisionjet including the strong vortex flow and the strong separation flow canbe generated, fine cavitation bubbles largely grow and new cavitationbubbles are generated in the collision jet. Accordingly, the cavitationbubbles having high collapse pressures can be collapsed on the surfaceof the structural member.

In accordance with the present invention, a preventive maintenancemethod of a structural member in a reactor pressure vessel for reducinga tensile residual stress on a surface thereof, has the steps ofimpinging a water jet from a nozzle onto a recess of a deflector tothereby change direction of flow of said water jet, and impinging thewater jet after being deflected onto the surface of the structuralmember.

In this case, since flow direction of a collision jet is opposed to thatof the water jet from the nozzle and a water-hammering effect becomesmuch stronger, a strong vortex flow and a strong separation flow aregenerated in the collision jet. Accordingly, the cavitation bubbleshaving high collapse pressures can be collapsed on the surface of thestructural member.

Preferably, the recess is in shape of cone with an apex angle of atleast 90° (preferably at least 120°) in a longitudinal cross sectionthereof.

Further, preferably, the structural member is a core shroud, an in-coremonitor housing or a water-level measuring nozzle.

In accordance with the present invention, a preventive maintenanceapparatus of a structural member in a reactor pressure vessel forreducing a tensile residual stress on a surface thereof, has a nozzlefor discharging a water jet into core water in a reactor pressurevessel, a deflector having a plane surface which is impinged by saidwater jet to change direction of flow of said water jet discharged fromthe nozzle, and a support maintaining a predetermined distance betweenthe nozzle and the plane surface of the deflector.

Preferably, the support maintains the distance between the nozzle andthe plane surface of the deflector at most 100 times (preferably at most50 times) as large as a hole diameter of the nozzle. In this case, thenozzle and the deflector supported by the support can access easily to anarrow space portion (or the inside of a tube with a small diameter).Further, a suitable distance between the nozzle and the plane surface ofthe deflector can be maintained certainly by the support.

Preferably also, the support maintains an angle (collision angle),formed between a central axis passing through an opening of the nozzleand the plane surface of the deflector, in a range of 10° to 90°,preferably in a range of 40° to 90°, more preferably in a range of 60°to 90°. In this case, a suitable collision angle can be maintainedcertainly by the support.

Preferably also, the support has one opening for discharging thedirection-changed flow of the water jet (collision jet), near the planesurface of the deflector. In this case, part of the collision jetflowing toward direction in which the opening is not provided, changesits flow direction toward the opening by making a second collision withan inner wall of the support and are discharged from the opening so asto collide with the surface of the structural member. Cavitation bubblesin the collision jet grow more largely by this second collision.Accordingly, the cavitation bubbles having high collapse pressures canbe collapsed on the surface of the structural member.

Preferably also, the support has openings for discharging thedirection-changed flow of the water jet near the plane surface of thedeflector, the openings being arranged in a peripheral direction withrespect to a central axis passing through an opening of the nozzle. Inthis case, surfaces corresponding to the openings can be treated with aWJP method simultaneously.

Preferably also, further has a pressurized water supply for supplyingpressurized water to the nozzle.

In accordance with the present invention, a preventive maintenanceapparatus of a structural member in a reactor pressure vessel forreducing a tensile residual stress on a surface thereof, has a nozzlefor discharging a water jet into core water in a reactor pressurevessel, a deflector having a recess which is impinged by the water jetto change direction of flow of the water jet discharged from the nozzle,and a support maintaining a predetermined distance between the nozzleand the recess of the deflector.

Preferably, the recess is in shape of cone with an apex angle of atleast 90° (preferably at least 120°) in a longitudinal cross sectionthereof.

Further, preferably, the support has openings for discharging thedirection-changed flow of the water jet near the recess of thedeflector, the openings being arranged in a peripheral direction withrespect to a central axis passing through an opening of the nozzle.

Further, preferably also, the recess of the deflector has spiral groovesor spiral projections for making a revolving flow of thedirection-changed flow of the water jet (collision jet) with respect tothe central axis passing through the opening of the nozzle. In thiscase, since the collision jets discharged from the openings of thesupport are given velocity components in the peripheral direction, thecollision jets being uniform in the peripheral direction can be formed.This collision jets are suitable to treat an inner surface of a tubularstructure with a WJP method.

Preferably also, further has a pressurized water supply for supplyingpressurized water to the nozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic longitudinal sectional view of a RPV in a statethat a top head of the RPV, a steam drier and a shroud head are removed.

FIG. 1B is a schematic configuration view showing a state that a WJPmethod of the present invention is applied to a vertical weld portion onan outer surface of a shroud.

FIG. 1C is a diagram showing a collision angle and a collision distance.

FIG. 1D is a diagram showing another example of a baffle body in FIG.1B.

FIG. 2 is a schematic configuration view showing a one-sided dischargingtype nozzle head of the present invention.

FIG. 3 is a diagram showing one example of an improvement effect of aresidual stress by using the one-sided discharging type nozzle headshown in FIG. 2.

FIG. 4A is a schematic longitudinal sectional view of a surrounding areanear a RPV in a state that a WJP method according to the presentinvention is applied to a vertical weld portion on an outer surface of ashroud (the first embodiment) and a horizontal weld portion of an ICMhousing (the third embodiment).

FIG. 4B is a top view of a surrounding area near a WJP main body duringexecuting the WJP in the first embodiment.

FIG. 5 is a flow chart showing execution steps of the WJP in the firstembodiment.

FIG. 6A is a schematic configuration view showing one example of afour-sided discharging type nozzle head according to the presentinvention.

FIG. 6B is an A—A cross sectional view of FIG. 6A.

FIG. 6C is a cross sectional view showing another example of afour-sided discharging type nozzle head according to the presentinvention.

FIG. 7A is a cross sectional view showing another example of afour-sided discharging type nozzle head according to the presentinvention.

FIG. 7B is a B—B cross sectional view of FIG. 7A.

FIG. 7C is a C—C cross sectional view of FIG. 7A.

FIG. 7D is a longitudinal sectional view showing another example of afour-sided discharging type nozzle head according to the presentinvention.

FIG. 8 is a longitudinal sectional view of a water-level measuringnozzle in a state that a WJP method according to the second embodimentof the present invention is applied to a weld portion of the water-levelmeasuring nozzle.

FIG. 9 is a longitudinal sectional view of a surrounding area near a RPVin the same state as FIG. 8.

FIG. 10 is a schematic configuration view showing another example of afour-sided discharging type nozzle head according to the presentinvention.

FIG. 11 is a schematic configuration view showing a state that a WJPmethod according to the third embodiment of the present invention isapplied to an inner surface of a horizontal weld portion of an ICMhousing.

FIG. 12 is a schematic configuration view showing a state that a WJPmethod according to the fourth embodiment of the present invention isapplied to an inner surface of

FIG. 13 is a diagram showing one example of an improvement effect of aresidual stress by using the four-sided discharging type nozzle headshown in FIG. 6A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A WJP method (preventive maintenance method) for a vertical weld portion(or line) of a core shroud (hereinafter referred to as “shroud”) in aboiling water reactor (BWR) according to the first embodiment of thepresent invention is explained by FIG. 1. In this embodiment, an objectof the WJP is the vertical weld portion on an outer surface of theshroud. The vertical weld portion is one of narrow space portions in aRPV (reactor pressure vessel).

FIG. 1A shows a schematic longitudinal sectional view of the RPV in astate that a top head of the RPV, a steam drier and a shroud head areremoved from the RPV. In this state, the RPV 13 is filled with corewater 22 and riser pipes 24, jet pumps 25, core cooling pipes 27, etc.are mounted in an annulus portion (a narrow space portion) between theshroud 23 and the RPV 13.

In some cases, the vertical weld portion 17 of the shroud 23 is locatednear the riser pipe 24, and a distance (a spatial width) between thevertical weld portion 17 and the riser pipe 24 is as narrow as about afew tens (20 to 30) mm. In a case that the spatial width is narrow likethis, it is impossible to direct a nozzle 4 to the vertical weld portion17 and to discharge a water jet (hereinafter referred to as “jet”) 3from the nozzle 4. FIG. 1A shows also an ICM housing mounted in a bottomhead 26 of the RPV 13.

FIG. 1B is a schematic configuration view which shows a state that theWJP method of the present invention is applied to the vertical weldportion on the outer surface of the shroud. In FIG. 1B, the riser pipe24 is not shown for simplicity. As shown in FIG. 1B, the nozzle 4 isinserted substantially in parallel to the outer surface of the shroud 23by moving a lifting means 6 using, for example, a fuel exchangerassisting hoist (not shown).

Pressurized water flows through a hole in the nozzle 4 and is dischargeddownward from an opening of the nozzle 4 as a jet 3. When the jet 3 isdischarged from the opening, cavitation bubbles 2 a are generated. Thisjet 3 containing cavitation bubbles 2 a collides with (or impinges on) aplane surface (hereinafter referred to as “a collision surface”) of abaffle body 5 a provided near the vertical weld portion 17.

The jet 3 changes direction and velocity of its flow by the collisionwith the collision surface of the baffle body 5 a, and collides with thevertical weld portion 17 as a collision jet 9 a. That is, the bafflebody 5 a is a deflector of the jet 3. Although it is omitted in FIG. 1B,practically, a relative position between the nozzle 4 and the collisionsurface of the baffle body 5 a is maintained by a support.

A distance between an end of the nozzle 4 and the collision surface 50of the baffle body 5 a is defined as a collision distance L as shown inFIG. 1C. Strictly, the collision distance L is a distance in a centralaxis 3 a passing through the opening of the nozzle 4. In thisembodiment, the collision distance L is set at most 100 times(preferably at most 50 times) as large as a hole diameter of the nozzle4. This hole diameter means a substantial diameter of the hole in thenozzle 4.

By arranging the nozzle 4 and the baffle body 5 a so as to meet theabove condition, the jet 3 collides with the collision surface 50 beforefine cavitation bubbles contained in the jet 3 become large. Therefore,since the amount (a ratio) of the cavitation bubbles collapsed by thecollision with the collision surface 50 is reduced and the jet 3collides with the collision surface 50 before its velocity becomes low,the collision jet 9 a including a strong vortex flow and a strongseparation flow is generated.

Accordingly, the fine cavitation bubbles, which are not collapsed by thecollision with the collision surface 50, grow in the collision jet 9 aand collapse at the vertical weld portion 17 with high collapsepressures, thereby a tensile residual stress of the vertical weldportion 17 can be reduced effectively.

If the collision distance L is set more than 100 times as large as thehole diameter of the nozzle 4, the amount (the ratio) of the cavitationbubbles collapsed by the collision with the collision surface 50 becomeslarge and the velocity of the jet 3 becomes low. Therefore, the amount(the ratio) of the cavitation bubbles contained in the collision jet 9 ais reduced and an improvement effect of the residual stress decreases.

As shown in FIG. 1C, an angle formed the central axis 3 a passingthrough the opening of the nozzle 4 and the collision surface 50 isdefined as a collision angle α. Strictly, the collision angle a is alower (smaller) angle of two angles formed the central axis 3 a and thecollision surface 50 on a plane 51 including both the central axis 3 aand a perpendicular line 50 a of the collision surface 50, theperpendicular line 50 a passing through an intersection point where thecentral axis 3 a crosses the collision surface 50. The collision angle ais an acute angle except a case that the central axis 3 a crossesperpendicularly the collision surface 50.

The collision angle α is needed to be at least 10°. When the jet 3collides with the collision surface 50, not only the collision jet 9 aflowing toward the vertical weld portion 17 but also, for example, acollision jet 9 b flowing opposite to the vertical weld portion 17 isgenerated.

If the collision angle α is set about 10°, since the collision surfacehas a steep slope (incline) to the vertical weld portion 17, a rate ofthe collision jet 9 a can be higher and a rate of the collision jet 9 bcan be lower in comparison with a case of a α<10°. In this case,however, the vortex flow and the separation flow in the collision jet 9a are not so strong because the water-hammering effect on the collisionsurface 50 is still weak. Therefore, a long period of time fordischarging the jet 3 is needed to attain a desired effect of improvingthe residual stress.

In this embodiment, the collision angle α is set in a range of 40° to90° (preferably in a range of 60° to 90°). In this case, since thewater-hammering effect on the collision surface 50 becomes strong, thestrong vortex flow and the strong separation flow can be generated inthe collision jet 9 a. Accordingly, it is possible to impinge thecollision jet 9 a containing the cavitation bubbles with the highcollapse pressures on the vertical weld portion 17, and also attain thedesired effect of improving the residual stress more effectively.

According to this embodiment, it is easy to indirectly impinge the jet 3on the vertical weld portion 17 without directing the nozzle 4 to thevertical weld portion 17. When the jet 3 collides with the collisionsurface 50, part of cavitation bubbles 2 a contained in the jet 3collapse due to an increase of a fluid pressure caused by thewater-hammering effect. But the remaining cavitation bubbles, which donot collapse on the collision surface 50, grow to the cavitation bubbleswith the high collapse pressures in the collision jet 9 a including thestrong vortex flow and the strong separation flow.

In the collision jet 9 a, in addition to the above mentioned growth ofthe remaining cavitation bubbles, new cavitation bubbles are alsogenerated and then grow. As a result, the collapse pressure of thecollision jet 9 a on the vertical weld portion 17 becomes higher, and itis possible to attain the effect of improving significantly the residualstress of the vertical weld portion 17.

FIG. 1D shows another example of the baffle body which is used forchanging the direction of the flow of the jet 3 in FIG. 1B. This bafflebody has a curved surface 5 d as the collision surface and jet guids 5d′ which are provided at both sides of the curved surface 5 d. In a caseof using this baffle body, the strong vortex flow and the strongseparation flow are generated in the collision jet 9 a, and thecollision jet 9 a containing the cavitation bubbles with the highcollapse pressures can collide with (impinge on) the vertical weldportion 17. Further, it is possible to reduce effectively the rate(amount) of collision jet except the collision jet 9 a flowing towardthe weld portion 17.

One example of a nozzle head, which can discharge a collision jet toalmost one direction, according to the present invention is explained byFIG. 2. FIG. 2 shows a schematic configuration view of a nozzle head 15a which is a one-sided discharging type and has a flow baffle 5 with anopening at one side. Hereinafter, this flow baffle is referred to as “aone-sided opening type flow baffle”.

This flow baffle 5 is formed into a cylindrical shape and has asquare-shaped opening 5 b which is formed by cutting out acircumferential part near one end portion of the cylinder. A baffle body5 a is removably engaged with the one end portion of the flow baffle 5at a position adjacent to the opening 5 b in such a manner that acollision jet 9 a passing through the opening 5 b collides with asurface to be treated. The nozzle head 15 a is constructed by engagingremovably a nozzle 4 with the other end portion of the flow baffle 5.

Since the baffle body 5 a is removably engaged with the flow baffle 5,when the baffle body 5 a is worn, it can be easily replaced with a newone. Therefore, reliability of execution of WJP can be maintained. Inthis nozzle head 15 a, a collision distance and a collision angle areset in the above-mentioned range.

In FIG. 2, the jet 3 collides with the collision surface of the bafflebody 5 a to change its flow direction, and the collision jet 9 adirectly collides with the surface to be treated. A collision jet 9 bflowing toward direction in which the opening 5 b is not provided,changes its flow direction toward the opening 5 b by making a secondcollision with an inner wall of the flow baffle 5, and are dischargedfrom the opening 5 b so as to make a third collision with the surface tobe treated. In this case, cavitation bubbles in the collision jet growmore largely by this second collision, and the collision jet canrestrictively collide with the surface to be treated.

Further, by making fine irregularities on the collision surface of thebaffle body 5 a, the cavitation bubbles grow largely by the collisionwith the collision surface having the fine irregularities. This growthof the cavitation bubbles can make a strong peening effect (a strongeffect of improving the residual stress) in cooperation with the abovementioned repeated collision.

In FIG. 2, it is possible to replace the cylindrical flow baffle with asquare pipe flow baffle. It is also possible to replace the planecollision surface with a curved surface as shown in FIG. 1C.

FIG. 3 shows one example of an improvement effect of the residual stressby using the one-sided discharging type nozzle head 15 a shown in FIG.2. The nozzle having an outer diameter of 30 mm and a hole diameter of 2mm is used. The baffle body 5 a is arranged so as to make the collisiondistance of 80 mm and the collision angle of 70°. The one-sided openingtype flow baffle 5 has the opening 5 b in a half circumferential part.

FIG. 3 shows a measurement result of the residual stress on a surface ofa strip-shaped (plate-shaped) test piece after executing the WJP to thetest piece using this nozzle head 15 a. The WJP is executed in acondition that the nozzle head is moving to a longitudinal direction(Y-direction) by keeping a distance between the nozzle head and thesurface of the test piece about 5 mm.

In FIG. 3, a vertical axis is a relative measurement value of theresidual stress, and a horizontal axis is a distance from a center line(Y-axis) of the test piece in a width direction (X-direction). Apositive residual stress is a tensile residual stress, and a negativeresidual stress is a compressive residual stress. The surface of thetest piece is subjected to surface grinding so as to have a tensileresidual stress of about 400 MPa as an initial residual stress. As shownin FIG. 3, the initial tensile residual stress is improved to thecompressive residual stress in a range in which the collision jetcollides with the surface of the test piece.

The first embodiment, in which the WJP method according to the presentinvention is applied to the vertical weld portion on the outer surfaceof the shroud in a BWR plant after at least the first operation cycle,is explained in more detail using FIG. 4 and FIG. 5. A WJP apparatushaving the one-sided discharging type nozzle head 15 a with thecylindrical flow baffle 5 is used. The collision distance and thecollision angle are set in the above-mentioned range, respectively.

FIG. 4 is a schematic longitudinal sectional view, which shows a stateof the WJP execution, of a surrounding area near the RPV. FIG. 4 alsoshows the third embodiment in which the WJP method according to thepresent invention is applied to a horizontal weld portion of an ICMhousing. FIG. 5 is a schematic flow chart which shows execution steps ofthe WJP In the first embodiment. Each stop is explained below accordingto the flow chart of FIG. 5.

(1) Disconnection: A top head of the RPV, a steam drier and a shroudhead are removed from the RPV. In this state, the RPV 13 and a reactorwell are filled with core water 22.

(2) Detection of weld line: A weld line detector (not shown) is loweredand set near an outer surface of the shroud using, for example, a fuelexchanger assisting hoist (hereinafter referred to as “assisting hoist”)21. A vertical weld portion (line) is detected by the weld linedetector.

(3) Confirmation of access route: While a monitor camera 30 is loweredusing, for example, the assisting hoist 21, an access route to the weldline 17, presence or absence of an obstacle to set a WJP main body 29,and the weld line 17 are confirmed by means of a monitor video 31. Aspatial distance between a riser pipe 24 and the shroud 23 is measuredto confirm that a nozzle head can be inserted into the space.

(4) Setting of WJP apparatus: A control panel 20 and a booster pump 18are disposed on an operation floor. The booster pump 18 is connected toa source water tank (not shown) by means of a water supply hose 19. Thebooster pump 18 is connected to the WJP main body 29 by means of ahigh-pressure hose 7. Wiring between these devices is laid out, andthese devices are adjusted.

(5) Setting of WJP main body: This step has next steps of a) to e).

a) Lowering: The WJP main body 29 is lowered by the assisting hoist 21to a specific height in a space between the shroud 23 and the RPV 13. Itis confirmed by the monitor camera 30 and the monitor video 31 that theWJP main body 29 is located in a suitable height.

b) Fixing: Upper and lower portions of the WJP main body 29 are fixed ona shroud's side and a RPV's side by a support 29 a and a support 29 b.

c) Extending nozzle arm: A nozzle head 15 a fixed at a top end of anozzle arm 33 is inserted between the shroud 23 and the riser pipe 24 byextending forwardly the nozzle arm 33.

d) Confirmation of position: A distance between the weld line 17 and thenozzle head 15 a and discharging direction are confirmed by the monitorcamera 30 and the monitor video 31.

e) Trial discharge of jet: A trial discharge of a collision jet 9 isperformed to confirm that the collision jet 9 collides with a desiredposition by the monitor camera 30 and the monitor video 31. It is thelast step for setting of the WJP main body 29.

(6) Execution of WJP: This step has next steps of a) to c).

a) Setting of execution conditions: A discharging pressure and a flowrate of the jet, and a moving speed and a moving range of the nozzlehead 15 a are set.

b) Discharge of jet: The collision jet 9 is discharged and the nozzlehead 15 a is moved in a vertical direction along the weld line 17 toexecute the WJP. This execution state of the WJP is confirmed by themonitor camera 30 and the monitor video 31.

In this state, the schematic longitudinal sectional view of thesurrounding area near the RPV is shown in FIG. 4A, and a top view of asurrounding area near the WJP main body 29 is shown in FIG. 4B.

c) Confirmation of execution of WJP: A state of a surrounding area nearthe weld line 17 after the execution of the WJP is confirmed by themonitor camera 30 and the monitor video 31 to terminate the execution ofthe WJP.

(7) Withdrawal of WJP main body: This step has next steps of a) to d).

a) Folding of nozzle arm: The nozzle arm 33 is folded to be contained inthe WJP main body 29.

b) Release of main body: The WJP main body 29 fixed between the shroud23 and the RPV 13 is released.

c) Confirmation of preparation for lifting: A termination of preparationfor lifting the WJP main body 29 is confirmed by the monitor camera 30and the monitor video 31.

d) Lifting of main body: The WJP main body 29 is lifted by the assistinghoist 21.

(8) Withdrawal of WJP apparatus: The connection between the booster pump18 and the source water tank by the water supply hose 19 and theconnection between the booster pomp 18 and the WJP main body 29 by thehigh pressure hose 7 are released, and the wiring between these devicesis removed. The apparatuses such as the WJP main body 29, the controlpanel 20, the booster pump 18, the high pressure hose 7 and the watersupply hose 19 are withdrawn.

(9) Withdrawal of monitor camera: The monitor camera 30 is withdrawn.

(10) Withdrawal of weld line detector: The weld line detector iswithdrawn to terminate the execution of the WJP.

(11) Synchronization: The shroud head, the steam drier, and the top headof the RPV are lowered and assembled to be restored.

By executing (applying) the WJP with the above steps to the verticalweld portion on the outer surface of the shroud in the RPV filled withthe core water, it is possible to collapse cavitation bubbles with highcollapse pressures on a surface of the vertical weld portion.Accordingly, the residual stress on the surface of the vertical weldportion can be improved and a damage such as the SCC can be prevented.

When the above WJP method is executed during an outage of the BWR plant,since the top head of the RPV, the steam drier and the shroud head arealready removed, the execution of the WJP is started from the step (2)and terminated at the step (9). The one-sided discharging type nozzlehead 15 a can be applied to axial weld lines on both inner and outersurfaces of a weld pipe. Of course, it can be applied to a weld pipewith no weld line.

One example of a four-sided discharging type nozzle head according tothe present invention is explained by FIG. 6. FIG. 6A shows a schematicconfiguration view of this nozzle head, and FIG. 6B shows an A—A crosssectional view of FIG. 6A. This nozzle head 15 b has a cylindrical flowbaffle 5 with four square openings 5 b which are arranged symmetricallyin a peripheral direction. Each of four supports 5 x forming theopenings 5 b has a square-shaped cross section.

A baffle body 5 a having a flat collision surface is removably engagedwith one end portion of the flow baffle 5 at a position adjacent to theopenings 5 b. A nozzle 4 is removably and rotatably engaged with theother end portion of the flow baffle 5. A collision angle is about 90°and a collision distance is set in the above-mentioned range. Since thebaffle body 5 a is removably engaged with the flow baffle 5, when thebaffle body 5 a is worn, it can be easily replaced with a new one.Therefore, reliability of execution of WJP can be maintained.

In this nozzle head 15 b, a jet 3 having cavitation bubbles collideswith the collision surface of the baffle body 5 a and is discharged fromthe four openings 5 b as four collision jets 9 a. Therefore, it ispossible to execute the WJP simultaneously to a plurality of objects tobe treated which are disposed opposite to the four openings 5 b. In thiscase, since velocity of the collision jets 9 a in an axial directionbecomes almost zero, a strong water-hammering effect and a turbulentflow are generated, and an vortex flow and a separation flow generatedin the collision jet become strong.

In this nozzle head 15 b, by making width of each opening 5 b wider, thecollision jets 9 a can be discharged in approximately radial directions.In this case, the nozzle head 15 b can make an almost omni-directionaldischarge which is suitable for executing the WJP to an entire innersurface of a cylinder. Therefore, by discharging the jet from thisnozzle head to a peripheral weld portion on an inner surface of such atube with a small diameter, it is possible to execute the WJPsimultaneously to the entire peripheral weld portion without rotatingthis nozzle head from outside. Also, by increasing the number of theopenings 5 b, the collision jets 9 a can be discharged in approximatelyradial directions.

Further, in this nozzle head 15 b, since the openings 5 b are madelonger in the axial direction, the jet 3 can draw water near theopenings 5 b. Therefore, since cavitation bubbles contained in the jet 3can grow largely before the collision with the baffle body 5 a, theimprovement effect of the residual stress by the collision jet becomeshigher.

Another example of a four-sided discharging type nozzle head accordingto the present invention is explained by FIG. 6C. FIG. 6C shows a crosssectional view which corresponds to the A—A cross sectional view of FIG.6A. In this nozzle head, each of four supports 5 x forming the openings5 b has curved sides as shown in FIG. 6C. As a result, the support 5 xhas an almost parallelogram-shaped cross section. The collision jets 9 abecome to have velocity components in both a radial direction and aperipheral direction by passing through this openings 5 b. That is, thecollision jets 9 a become a revolving flow.

In this nozzle head, since the collision jets 9 a become the revolvingflow, the collision jets 9 a can go around to portions which are notdisposed opposite to the openings. Further, the nozzle 4 is not rotatedbut the flow baffle 5 is rotated on its axis by a reaction force to therevolving flows. Therefore, this nozzle head is more suitable forexecuting the WJP to the entire inner surface of the cylinder than thatshown in FIG. 6B. That is, this nozzle head can make an almostomni-directional discharge of the collision jets.

Another example of a four-sided discharging type nozzle head accordingto the present invention is explained by FIG. 7. FIG. 7A shows a crosssectional view which corresponds to FIG. 6B. FIG. 7B and FIG. 7C show aB—B cross sectional view and a C—C cross sectional view of FIG. 7A,respectively. The other elements of this nozzle head are almost the sameas FIG. 6A. As shown in FIG. 7A, this nozzle head has a collisionsurface with four spiral grooves 5 c which are symmetrical with respectto an central axis of the collision surface. As shown in FIG. 7C, eachgroove 5 c has a V-shaped cross section.

In this nozzle head, the collision jet 9 a discharged from the openingis given a velocity component in a peripheral direction by the groove 5c. That is, the collision jets 9 a become a revolving flow. As a result,the collision jets 9 a can go around to portions which are not disposedopposite to the openings. Therefore, this nozzle head is also suitablefor executing the WJP to the entire inner surface of the cylinder.Further, if the spiral grooves 5 c are replaced with spiral projections,the same effect can be attained.

In FIG. 7A, the spiral grooves 5 c are originated from positions whichare separated from the central axis of the collision surface. If thespiral grooves 5 c are originated from the central axis of the collisionsurface, since vortex flows and separation flows contained in thecollision jets 9 a become stronger, the collision jets can becomecollision jets containing cavitation bubbles with high collapsepressures. Therefore, higher improvement effect of the residual stresscan be attained.

Further, by combining the spiral grooves 5 c with the supports 5 x shownin FIG. 6C, the peripheral velocity component of the collision jet 9 abecomes higher and the rotation speed of the flow baffle 5 on its axisalso becomes higher. Therefore, the improvement effect of the residualstress can be attained more effectively.

Another example of a four-sided discharging type nozzle head accordingto the present invention is explained by FIG. 7D. FIG. 7D shows alongitudinal sectional view which corresponds to FIG. 7B. The otherelements of this nozzle head are almost the same as FIG. 6A.

This nozzle head has a recessed baffle body 5 a which has a recess witha concave cross section as the collision surface. The recess is in shapeof cone with an apex angle β of at least 90° (preferably at least 120°)in a longitudinal cross section thereof. When a jet 3 collides with thecollision surface, velocity of the jet 3 in a collision direction (adownward direction in FIG. 7D) becomes zero on the collision surface,and then the jet 3 changes to a collision jet 9 a with a velocitycomponent in direction (an upward direction in FIG. 7D) opposed to thatof the jet 3.

Since a change in velocity from the jet 3 to the collision jet 9 becomeslarge by setting the apex angle β in the above range, a water-hammeringeffect occurs strongly on the collision surface. Therefore, part ofcavitation bubbles collapse strongly on the collision surface. Theremaining cavitation bubbles, which are not collapsed on the collisionsurface, grow in a strong vortex flow and a strong separation flowincluded in the collision flow 9 a, and are discharged.

Also, in this nozzle head, by forming spiral grooves (or spiralprojections) as shown in FIG. 7A on the collision surface, it ispossible to give a revolving flow to the collision jet 9 a and alsogenerate the vortex flow and the separation flow more strongly. As aresult, an improvement effect of the residual stress which is high andalmost uniform in the peripheral direction can be obtained.

As a modification of FIG. 7D, the collision surface can be formed into aprojecting surface (shape). In this case, the top of the projectingsurface breaks a central flow in the jet 3 and generates cavitationbubbles. Further, it becomes easy to form grooves (or projections) likeFIG. 7A on the collision surface by machining.

The second embodiment, in which the WJP method according to the presentinvention is applied to a weld portion of a water-level measuring nozzlein a BWR, is explained using FIG. 8. FIG. 8 is a longitudinal sectionalview which shows a state that a nozzle head 15 b is set in a water-levelmeasuring nozzle 35. An object of the WJP in this embodiment is a weldportion 38 between a nozzle 36 and a safe end 37 in the water-levelmeasuring nozzle 35 mounted in a RPV 13. The nozzle head 15 b shown inFIG. 7D is used in this embodiment.

A central flow (a flow near a central axis) in a jet 3 changes its flowdirection by a collision with a central portion of a recessed surface(collision surface) and then flows along the recessed surface, thereby astrong turbulent flow is generated by interference between thedirection-changed flow and an outer flow in the jet 3. A collision jetgenerated like this flows toward the RPV 13 (a right side in FIG. 8) inthe water-level measuring nozzle 35, and is finally discharged into theRPV 13 because a leading end of the water-level measuring nozzle 35 isclosed with a valve 37 a.

An apparatus used for execution of the WJP to the weld portion 38 in thewater-level measuring nozzle 35 is explained using FIG. 9. Thisapparatus has a nozzle head drive unit 39 for moving the nozzle head 15b to an object to be treated, a frame 40 for supporting the nozzle headdrive unit 39 at a level of the water-level measuring nozzle 35, ahigh-pressure hose 42 and a booster pump 43 for supplying pressurizedwater to a nozzle 4, a water supply hose 44 for supplying water to thebooster pump 43, and a control panel 45 for controlling the nozzle headdrive unit 39 and the booster pump 43.

The WJP is executed using the above apparatus in accordance with thefollowing steps.

(1) Disconnection: A top head of the RPV, a steam drier, a shroud headand fuel assemblies are removed from the RPV. In this state, the RPV 13and a reactor well are filled with core water 22.

(2) Setting of nozzle head drive unit: The nozzle head drive unit 39 ismounted on the frame 40. The nozzle head drive unit 39 is lowered in theframe 40 by an assisting hoist 21, and is set at a position of thewater-level measuring nozzle 35.

(3) Preparation for execution of WJP: This step has next steps of a) toc).

a) Setting of WJP apparatus: The nozzle head 15 b mounted at a top endof the nozzle head drive unit 39 is inserted in the water-levelmeasuring nozzle 35. The control panel 45 and the booster pump 43 aredisposed on an operation floor. The booster pump 43 is connected to asource water tank 46 by the water-supply hose 44. The booster pump 43 isconnected to a WJP main body by the high-pressure hose 42. Wiringbetween these devices are laid out, and these devices are adjusted.

b) Setting of execution conditions: A flow rate and a discharging period(time) of the jet, a moving speed and a moving range of the nozzle headin an axial direction, and a turning speed and a turning range of thenozzle head in a peripheral direction are set.

c) Confirmation of operation of apparatus: The nozzle head 15 b is movedaccording to the setting conditions in a head 15 b is moved according tothe setting conditions in a state in which the jet is not discharged, toconfirm whether or not the execution range is suitable, the nozzle head15 b is smoothly moved, and the like.

(4) Execution of WJP: A jet is discharged to start the execution of theWJP.

(5) Confirmation of execution of WJP: This step has next steps of a) tob).

a) Removal of nozzle head: The nozzle head 15 b is removed from thewater-level measuring nozzle 35.

b) Confirmation of execution of WJP: A monitor camera 47 is inserted inthe water-level measuring nozzle 35. It is confirmed by a monitor TV 48that the WJP is suitably executed. The suitably executed state isrecorded in a monitor video 49.

(6) Withdrawal of WJP apparatus: This step has next steps of a) to b).

a) Withdrawal of monitor camera: The monitor camera 48 is removed fromthe water-level measuring nozzle 35 to be withdrawn.

b) Withdrawal of WJP apparatus: Piping and wiring between the abovedevices are removed. The devices, pipes for piping, and wires for wiringare withdrawn.

(7) Synchronization: The fuel assemblies, the shroud head, the steamdrier, and the top head of the RPV are lowered and assembled to berestored.

By executing (applying) the WJP with the above steps to the weld portionof the water-level measuring nozzle in the RPV filled with the corewater, it is possible to collapse cavitation bubbles with high collapsepressures on a surface of the weld portion. Accordingly, the residualstress on the surface of the weld portion can be improved and a damagesuch as the SCC can be prevented.

Another example of a four-sided discharging type nozzle head accordingto the present invention is explained by FIG. 10. FIG. 10 shows aschematic configuration view of this nozzle head. This nozzle head has aturning vane 5 d adjacent to the baffle body 5 a on an opposite side tothe nozzle. The turning vane 5 d and the baffle body 5 a have the samecentral axis. That is, this nozzle head has the flow baffle 5 with theturning vane 5 d. The other elements of this nozzle head are almost thesame as FIG. 6A.

In this case, the turning vane 5 d is turned by the collision jet whichchanged its flow direction by the collision with an object to betreated, and this rotation of the turning vane 5 d assists a rotation ofthe baffle body 5 a on its axis.

The third embodiment, in which the WJP method according to the presentinvention is applied to an inner surface of a horizontal weld portion(or line) of an ICM housing in a BWR, is explained using FIG. 11. FIG.11 is a schematic configuration view which shows a state that a nozzlehead 15 c with a back-flow obstructive plate 10 is set at the innersurface of the horizontal weld portion 17 a of the ICM housing 1. Asshown in FIG. 4A, the TCM housing 1 pierces a bottom head 26 of the RPV13 and is fixed to the bottom head 26.

The nozzle head 15 c corresponds to the nozzle head 15 b (shown in FIG.8) to which the back-flow obstructive plate 10 is added on a nozzleside. Since the nozzle head 15 c has the back-flow obstructive plate 10,a sealing portion located at a lower end of the ICM housing 1 isprotected for sealing water.

In this nozzle head 15 c, the collision jet, which changed its flowdirection opposite to an initial flow direction of the jet 3 by thecollision with the buffle body 5 a, can change (be repelled) its flowdirection to the initial flow direction by a collision with theback-flow obstructive plate 10. Interference between this repelledcollision jet and the initial collision jet makes a turbulent flow, andthis turbulent flow can make the peening effect higher.

As shown in FIG. 11, an apparatus used for execution of the WJP to thehorizontal weld portion 17 a of the TCM housing 1 has a lifting shaft 6a with a lifting guide 14 mounting the nozzle head 15 c, a nozzle headdrive unit 16 having a nozzle rotating means 16 a provided at a lowerend of the lifting shaft 6 a and a nozzle lifter 16 b, a high-pressurehose 7 and a booster pump 18 for supplying pressurized water to a nozzle4, a water supply hose 19 for supplying water to the booster pump 18,and a control panel 20 for controlling the nozzle head drive unit 16 andthe booster pump 18.

The WJP is executed using the above apparatus in accordance with thefollowing steps.

(1) Disconnection: A top head of the RPV, a steam drier, a shroud head,fuel assemblies and control rods are removed from the RPV. In thisstate, the RPV 13 and a reactor well are filled with core water 22.

(2) Water sealing for upper end of ICM guide tube: The upper end of theTCM guide tube above the ICM housing 1 shown in FIG. 1A is plugged forsealing water.

(3) Removal of ICM detector: The ICM detector (not shown) contained inthe ICM guide tube is removed from the lower end of the ICM housing 1.

(4) Confirmation of welding position: An ultrasonic sensor (not shown)or the like is inserted from the lower end of the ICM housing 1 toconfirm a position of the horizontal weld portion 17 a and an executionrange of the WJP.

(5) Preparation for execution of WJP: This step has next steps of a) toe).

a) Setting of WJP apparatus: A nozzle drive shaft 16 c mounting thenozzle head 15 c at the leading end is inserted in the ICM housing 1.The nozzle head drive unit 16, control panel 20 and the booster pump 18are disposed as shown in FIG. 4A. The booster pump 18 is connected to asource water tank (not shown) by the water supply hose 19. The boosterpump 18 is connected to a WJP main body by the high-pressure hose 7.Wiring between these devices is laid out, and these devices areadjusted.

b) Setting of execution conditions: A flow rate and a discharging period(time) of the jet, a moving speed and a moving range of the nozzle headin an axial direction, and a turning speed and a turning range of thenozzle head in a peripheral direction are set.

c) Confirmation of operation of apparatus: The nozzle head 15 c is movedaccording to the setting conditions in a state in which the jet is notdischarged, to confirm whether or not the execution range is suitable,the nozzle head 15 c is smoothly moved, and the like.

d) Release of sealing of upper end of ICM guide tube: The plugging ofthe upper end of the ICM guide tube is released.

e) Trial discharge of jet: The trial discharge of the jet 3 is performedfor conforming looseness of pipes, a vibrational state, and the like. Inthis way, the setting of the WJP apparatus is terminated.

(6) Execution of WJP: The jet 3 is discharged to start execution of theWJP to the horizontal weld portion 17 a.

(7) Confirmation of execution of WJP: This stop has next steps of a) tod).

a) sealing for upper end of ICM guide tube: The upper end of the ICMguide tube is plugged for sealing water.

b) Removal of nozzle drive shaft: The nozzle drive shaft 16 c is removedfrom the ICM housing 1.

c) Setting of monitor camera: The monitor camera 30 is inserted in theICM housing 1 and is set in co-operation with the monitor video 31.

d) Confirmation of execution of WJP: It is confirmed that the WJP issuitably executed by the monitor camera 30.

(8) Withdrawal of WJP apparatus: This step has next steps of a) to b).

a) Withdrawal of monitor camera: The monitor camera 30 is removed fromthe ICM housing 1 to be withdrawn.

b) Withdrawal of WJP apparatus: The wiring and piping between the abovedevices are removed, and the devices, pipes for piping, and wires forwiring are withdrawn.

(9) Mounting of ICM detector: The ICM detector is inserted from thelower end of the ICM housing 1 to be mounted.

(10) Release of sealing of upper end of ICM guide tube: The plugging ofthe upper end of the ICM guide tube is released. In this way, theexecution of the WJP is terminated.

(11) Synchronization: The fuel assemblies, the control rods, the shroudhead, the steam drier and the top head of the RPV are lowered andassembled to be restored.

By executing (applying) the WJP with the above steps to the horizontalweld portion of the ICM housing in the RPV filled with the core water,it is possible to collapse cavitation bubbles with high collapsepressures on a surface of the horizontal weld portion. Accordingly, theresidual stress on the surface of the horizontal weld portion can beimproved and a damage such as the SCC can be prevented.

In this embodiment, the nozzle head 15 c with the back-flow obstructiveplate 10 is used. However, the nozzle head 15 a and 15 b shown in FIGS.2 and 6 are also can be used in the above steps.

The fourth embodiment, in which the WJP method according to the presentinvention is applied to an inner surface of a horizontal weld portion ofan ICM housing in a BWR, is explained using FIG. 12. In the thirdembodiment, the nozzle head is inserted from the lower end of the ICMhousing, however, in this embodiment, the nozzle head is inserted fromthe upper end of the ICM housing. In this embodimen. The nozzle head 15b shown in FIG. 8 is used. FIG. 12 is a schematically constructionalview which shows a state that the nozzle head 15 b is set at the innersurface of the horizontal weld portion 17 a of the ICM housing 1.

Executing steps of WJP according to this embodiment is explained below.

(1) Disconnection: A top head of the RPV, a steam drier, a shroud head,fuel assemblies and control rods are removed from the RPV. In thisstate, the RPV 13 and a reactor well are filled with core water 22.

(2) Water sealing for upper end of ICM guide tube and removal of ICMdetector: This step has next steps of a) to b).

a) The upper end of the ICM guide tube above the ICM housing 1 shown inFIG. 1A is plugged for sealing water. In FIG. 12, 34 is a core support.

b) The ICM detector (not shown) contained in the ICM guide tube 1 a isremoved from the lower end of the ICM housing 1.

(3) Water sealing for lower end of ICM housing and release of sealingfor upper end of ICM guide tube: This step has next steps of a) to b).

a) A closing flange 32 is mounted at the lower end of the ICM housing 1for sealing water.

b) The plugging of the upper end of the ICM guide tube is released.

(4) Confirmation of welding position: An ultrasonic sensor (not shown)or the like is inserted from the upper end of the ICM guide tube 1 a toconfirm a position of the horizontal weld portion 17 a and an executionrange of the WJP.

(5) Preparation for execution of WJP: This step has next steps of a) tod).

a) Setting of WJP apparatus: A lifting shaft 6 a mounting the nozzlehead 15 b at the leading end is inserted in the ICM housing 1 from anupper side. A nozzle head drive unit 16, a control panel 20 and abooster pump 18 are disposed. The booster pump 18 is connected to asource water tank (not shown) by the water supply hose 19. The boosterpump 18 is connected to a WJP main body by a high-pressure hose 7.Wiring between these devices is laid out, and these devices areadjusted. The arrangement of these devices in this case aresubstantially the same as those shown in FIG. 4A. Therefore, theexplanation thereof is omitted.

b) Setting of execution conditions: A flow rate and a discharging period(time) of the jet, a moving speed and a moving range of the nozzle headin an axial direction, and a turning speed and a turning range of thenozzle head in a peripheral direction are set.

c) Confirmation of operation of apparatus: The nozzle head 15 b is movedaccording to the setting conditions in a state in which the jet is notdischarged, to confirm whether or not the execution range is suitable,the nozzle head 15 c is smoothly moved, and the like.

d) Trial discharge of jet: The trial discharge of the jet 3 is performedfor conforming looseness of pipes, a vibrational state, and the like. Inthis way, the setting of the WJP apparatus is terminated.

(6) Execution of WJP: The jet 3 is discharged to start execution of theWJP to the horizontal weld portion 17 a.

(7) Confirmation of execution of WJP: This step has next steps of a) toc).

a) Removal of nozzle drive shaft: The nozzle drive shaft (not shown) isremoved from the ICM housing 1.

b) Setting of monitor camera: The monitor camera is inserted in the ICMhousing 1 and is set in co-operation with the monitor video.

c) Confirmation of execution of WJP: It is confirmed that the WJP issuitably executed by the monitor camera.

(8) Withdrawal of WJP apparatus: This step has next steps of a) to b).

a) Withdrawal of monitor camera: The monitor camera is removed from theICM housing 1 to be withdrawn.

b) Withdrawal of WJP apparatus: The wiring and piping between the abovedevices are removed, and the devices, pipes for piping, and wires forwiring are withdrawn.

(9) Water sealing for upper end of ICM guide tube, mounting of ICMdetector, and release of sealing of upper end of ICM guide tube: Thisstep has next steps of a) to c).

a) The upper end of the ICM guide tube 1 a is plugged for sealing water.

b) The closing flange 32 at the lower end of the ICM housing 1 isremoved, and the ICM detector is inserted to be mounted.

c) The plugging of the upper end of the ICM guide tube 1 a is released.In this way, the execution of the WJP is terminated.

(10) Synchronization: The fuel assemblies, the control rods, the shroudhead, the steam drier and the top head of the RPV are lowered andassembled to be restored.

In this embodiment, since the nozzle head 15 b having the baffle body 5a with the recessed surface (collision surface) is used, a central flowin the jet 3 changes its flow direction by the collision with a centralportion of the recessed surface and then flows along the recessedsurface, thereby a strong turbulent flow is generated by interferencebetween the direction-changed flow and an outer flow in the jet 3. Acollision jet generated like this flows upward in the ICM housing 1, andis finally discharged into the RPV 13 because the closing flange 32 ismounted at the lower end of the ICM housing 1.

Since the collapse pressures of the cavitation bubbles become higher bya strong turbulent flow generated near the recessed surface, a highimprovement effect of the residual stress can be obtained. Accordingly,in this embodiment, the residual stress on the surface of the horizontalweld portion of the ICM housing can be improved and damage such as theSCC can be prevented like in the third embodiment.

FIG. 13 shows one example of the improvement effect of the residualstress by using the four-sided discharging type nozzle head 15 b shownin FIG. 6A. The nozzle 4 having an outer diameter of 30 mm and a holediameter of 2 mm is used. The baffle body 5 a is arranged so as to makethe collision distance of 80 mm and the collision angle of 90°.

FIG. 13 shows a measurement result of the residual stress on an innersurface of a test tube with an inner diameter of 38 mm after executingthe WJP to the inner surface of the test tube using this nozzle head 15b. The WJP is executed in a condition that the nozzle head is moving toan axial direction (Z-direction) of the test tube.

In FIG. 13, a vertical axis is a relative measurement value of theresidual stress, and a horizontal axis is a distance from a centerposition in an executing region of WJP in the Z-direction. A positiveresidual stress is a tensile residual stress, and a negative residualstress is a compressive residual stress. The test tube is divided intothree pieces, and its surface is subjected to surface grinding so as tohave a tensile residual stress of about 400 MPa as an initial residualstress.

As shown in FIG. 13, the initial tensile residual stress is improved tothe compressive residual stress by executing the WJP. Since it is knownthat no SCC and no fatigue fracture occur under the compressive stress,it is possible to prevent the SCC and the fatigue fracture by applyingthe above-mentioned WJP in accordance with the present invention.

While the WJP methods according to the present invention are applied tothe structural members in the RPV in the above-mentioned embodiments,objects applied by these WJP methods are not limited in this. That is,these WJP methods can be applied to tubes in a nuclear plant, generalindustrial machines and ships.

What is claimed is:
 1. A preventive maintenance method of a structuralmember in a reactor pressure vessel for reducing a tensile residualstress on a surface thereof comprising the steps of: impinging a waterjet from a nozzle onto a plane surface of a deflector to thereby changedirection of flow of said water jet; and impinging the water jet afterbeing deflected directly onto the surface of the structural member to betreated.
 2. A preventive maintenance method of a structural member in areactor pressure vessel according to claim 1, wherein a distance betweenthe nozzle and said plane surface of the deflector is at most 100 timesas large as a hole diameter of the nozzle.
 3. A preventive maintenancemethod of a structural member in a reactor pressure vessel according toclaim 2, wherein said distance is at most 50 times as large as the holediameter of the nozzle.
 4. A preventive maintenance method of astructural member in a reactor pressure vessel according to claim 1,wherein an angle formed between a central axis passing through anopening of the nozzle and said plane surface of the deflector is in arange of 10° to 90°.
 5. A preventive maintenance method of a structuralmember in a reactor pressure vessel according to claim 4, wherein saidangle is in a range of 40° to 90°.
 6. A preventive maintenance method ofa structural member in a reactor pressure vessel according to claim 5,wherein said angle is in a range of 60° to 90°.
 7. A preventivemaintenance method of a structural member in a reactor pressure vesselfor reducing a tensile residual stress on a surface thereof, comprisingthe steps of: impinging a water jet from a nozzle onto a recess of adeflector to thereby change direction of flow of said water jet; andimpinging the water jet after being deflected directly onto the surfaceof the structural member to be treated.
 8. A preventive maintenancemethod of a structural member in a reactor pressure vessel according toclaim 7, wherein said recess is in shape of cone with an apex angle ofat least 90° in a longitudinal cross section thereof.
 9. A preventivemaintenance method of a structural member in a reactor pressure vesselaccording to claim 8, wherein said apex angle is at least 120°.
 10. Apreventive maintenance method of a structural member in a reactorpressure vessel according to claim 9, wherein said structural member isa core shroud, an in-core monitor housing or a water-level measuringnozzle.